This invention relates to a photomultiplier tube (PMT) which detects low-level incident light by electron multiplication. More particularly, it relates to a PMT that detects the incident position of low-level light by means of a multi-anode array.
PMTs of the type contemplated by the present invention are used, for example, in radiation detectors in positron computed tomography (CT) apparatus. A radiation detector is generally composed of discrete crystals and a single PMT. The crystals emit light upon illumination of a radiation and the emitted light is subjected to photoelectric conversion and electron multiplication. The anodes in the PMT detect the incident position of the radiation. FIGS. 7 and 8 show the PMTs used in the radiation detectors, which are described in IEEE Transactions on Nuclear Science, Vol. 36, No. 1, February, 1989.
Part (a) of FIG. 7 is a side view of a multi-segment PMT, part (b) is its front view, and part (c) is its plan view. PMT generally indicated by 5 is composed of four separate segments 1-4 and the light incident on a photocathode 6 is intensified in each of the segments 1-4. In other words, the photoelectrons generated by photoelectric conversion from the photocathode 6 are multiplied by secondary emission through the sequence of dynodes 7-16 so that they can be detected by an anode 17. The incident position of light can be identified by knowing which of the four segments 1-4 contains the anode 17 having detected the photoelectrons.
FIG. 8 shows the construction of a mosaic BGO radiation detector that is composed of a plurality of crystals 21 and a single position-sensitive PMT 22. When a certain crystal 21 is illuminated with a .gamma.-ray, photons are generated and pass through a window 23 to strike a photocathode 24. The resulting photoelectrons are multiplied by mesh dynodes 25 and the last dynode 26, and the incident position of the .gamma.-ray is detected by a multi-wired anode 27 of an X/Y-crossed configuration.
The conventional radiation detectors described above are economical, since they use only one PMT 5 or 22 and more than one detector need not be used. However, in order to detect the entry position of incident light with high precision, a single PMT must be divided into many segments. As for the PMT 5 shown in FIG. 7, it is very difficult to separate a single PMT into many segments by the current electron tube technology. As for the PMT 22 shown in FIG. 8, it is possible to detect the incident position of light with fairly high precision but this requires a complex computing circuit for determining the incident position. As a result, the response speed of the detector slows down and the count rate of incident radiations decreases.